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Abstract With the exponential increase of wireless communication services that need high data rates/throughput to satisfy customers’ needs

Abstract
With the exponential increase of wireless communication services that need high data rates/throughput to satisfy customers’ needs. 5G mobile networks are expected to achieve high data rates and high network capacities with a very low latency to meet the future mobile networks demands. However, it presents a great challenge to raise the needed capacity for mobile backhauling. So, much more transmission systems and creative solutions are required to support this growth. Millimeter wave and microwave play a part in the mobile traffic backhaul as deploying optical fiber to every small cell is considered highly cost for mobile operators. The purpose of this thesis is to show how combining microwave and millimeter wave line-of-sight point-to-point links can meet 5G requirements especially high data rates and ultra-low latency, and also increase the backhauling link range and service availability. This solution maximizes spectrum efficiency, with great improvement in the capacity levels of wireless backhaul, while accelerating the much needed shift toward the use of higher frequency bands (millimeter wave) because of the higher bandwidth availability and low utilized frequency bands.

1. Introduction
1.1 Motivation

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Mobile traffic has much increased due to the increasing number of devices accessing mobile wireless networks, high speed applications and machine type communication. The fifth generations of cellular mobile communications are intended to achieve the extreme requirements for mobile wireless communications in the next decade.

Figure 1.1: The evolution of mobile communications requirements in the next decade 1.
Existing cellular mobile networks were built with wireless links (microwave links) and wired links (fiber or copper) cannot meet with the capacity, latency, reliability, and cost effectiveness required for the 5G cellular mobile networks. Mobile wireless networks should expand greatly their capacities for supporting high-density mobile users with high-speed throughput applications and services, so networks are getting denser than before. This results in a larger number of smaller cells which means much massive data traffic. The massive data traffic shall be connected through the mobile backhaul network with extreme requirements in terms of capacity, latency, reliability, energy efficiency, and cost effectiveness to the core network. Millimeter wave operating around 80 GHz (E-Band) for example can provide high throughput with very high frequency reuse by utilizing very narrow beams.

In this thesis, we will investigate feasibility, advantages, and challenges of future wireless communications backhauling over the millimeter wave frequencies then we will discuss line-of-sight point-to-point link bonding / carrier aggregation of different carriers and frequency band combinations to meet the extreme requirements of the future 5G backhaul networks.

1.2 Purpose and Scope

With the new broadband wireless applications and services and next fifth generation mobile systems introduction, and huge customers’ demand, challenges are being placed on mobile backhaul infrastructure. As cost-effective alternatives to fiber mobile backhaul, high speed and long distance wireless backhauling links are becoming increasingly attractive. However, there are significant technical challenges such as how to achieve higher transmission capacity and extend transmission distance range. For long distance range wireless backhaul, the major challenge is to achieve high capacity/throughput at microwave frequencies. While for high capacity wireless backhaul, the major challenge is to achieve long distance backhaul at millimeter wave frequencies. Therefore, we purpose dual band solution combining both microwave and millimeter wave backhaul to provide up to 10 Gbps data rates over several kilometers range. Combining/Bonding multiple carriers in the same or different frequency bands into a single virtual link referred as Multiband or Carrier Aggregation or radio link bonding, will provide the benefits of lower frequency availability along higher frequency band capacity. This new wireless transmission solution will can significantly increase transmission capacity, availability and leading to an efficient use of the available microwave and millimeter wave spectrum.

1.3 Structure of the thesis

Chapter 2 will introduce the necessary background knowledge of wireless cellular communication for our research and chapter 3 provides the basic theory about mobile backhaul, technologies and challenges. Chapter 4 focuses on millimeter wave and microwave Backhauling design, challenges, attenuation and propagation losses. Chapter 5 contains the technical study of multiband / carrier aggregation concept, proposes dual band solution, and contains complete link budget and performance simulation and results for a dual band solution example. Finally, conclusions are given along with proposals for further research.

2. Mobile networks evolution

2.1. Introduction

Mobile communication has achieved extreme commercial and innovation success over several generations of evolution. The exponential growth of data traffic, great increase of number of connection and continuous emergence of new services are main drivers of future mobile networks. Mobile communications have developed from analog voice calls to the existing digital technologies, which are able to provide many services with high data rate. This speed development in mobile communications as well as the increasing number of mobile internet mobile devices, leads to the exponential growth in broadband internet traffic. This chapter will introduce the mobile networks evolution path towards the fifth generations of cellular mobile communications. The fifth generations of cellular mobile communications is not yet standardized, and it is still in pre-standardization phase. Large players in the cellular mobile communications industry are investing so much on the research of the next generation mobile networks which are expected to launch commercially soon.

2.2. Challenges and problems of Existing Cellular Mobile Networks

The increase in demand for wireless broadband services requires faster throughput, and higher capacity networks, ultra-reliable and ultra-low latency connectivity that can deliver high definition video and new advanced applications such as Internet of Things and Machine type communications. This enormous number of connected devices will produce a huge amount of data traffic, which will be the main challenge in the evolution of the next-generation networks. Therefore, the critical goal is to satisfy the exponential rise in the number of connections and throughout in mobile broadband communications.

2.3. The Fifth Generation (5G)

The idea of 5th generation mobile networks is to provide advanced networks satisfy the requirements of year 2020 and beyond in terms of throughout, latency and number of connections. The fifth generation of cellular mobile communications system will provide a better performance than the previous generations. The radio frequency channels already used for the third generation networks and the fourth generation networks are congested, so the fifth generation is expected to use higher along with much wider frequency bands channels than the earlier generations to deliver the faster data rates and high throughputs. The fifth generation networks are expected to introduce great enhancements in spectral efficiency compared with now day’s networks. The capacity upgrade can be achieved by wider frequency channels, and higher and efficient modulation and coding schemes. 6 7
The overall technical aims of the fifth generation cellular mobile networks are to support the following performance enhancement:
• higher mobile data throughput
• higher end-user data rates
• longer battery life of low-power devices
• lower End-to-End (E2E) round trip latency
• Massive connectivity: 10 to 100x number of connected devices

2.8. Conclusions

In this chapter, the history of the cellular mobile networks evolution was briefly summarized. With the increase in the number of mobile devices, the current fourth generation cellular mobile networks suffers from several issues and challenges to meet the extreme requirements of the future cellular mobile networks. As a result, the evolution to the fifth generation cellular mobile generation is mandatory and the extreme performance requirements of fifth generation cellular mobile communication systems was defined in terms of capacity, data rate, latency, spectral and energy efficiency. Cellular mobile networks densification with large numbers of macro base stations and small cells is considered as an important solution for higher throughput and better coverage which triggered many studies and researches on new technologies to meet the high performance needs.

3. Millimeter wave and Microwave Backhauling

3.1. Introduction

With the evolution of cellular mobile networks, the demand of higher capacity, greater number of connections, better radio coverage and customer experience are driving the evolution for transmission network technologies, especially for the “Last mile backhaul”. In mobile communication networks, the mobile backhaul is the communication link between a base station (eNodeB) and the mobile core nodes. Mobile networks backhaul uses optical fiber, copper line or microwave links to carry the mobile traffic, the choice between them is depending on the required capacity, geographical terrain and base station location. Due to the increasing of capacity required to meet high speed data services, the backhaul network is under enormous pressure especially in urban regions. Mobile backhauling is considered as a key enabler and also a showstopper for 5th cellular mobile generation systems.
3.2. Mobile Backhaul/Fronthaul

The backhaul (or generally referred to as transport network) in cellular mobile networks, is the communication link (wired and/or wireless) between a base station (eNodeB) and the core network and usually consists of optical fiber, copper lines, microwave, millimetre wave and sometimes satellite links. Usually a number of base stations can be connected to the core network via a hub station or an aggregation site.
C-RAN concept was introduced as the key architecture for future cellular mobile networks, which consists of fronthaul and backhaul, as illustrated in Fig. 3.2 9. C?RAN separates baseband processing units (BBUs) from radio front?ends known as remote radio units (RRUs). In the C?RAN network architecture, the backhaul networks connect the remote radio head (RRH) directly to the baseband unit (BBU), or to an intermediate aggregation point which is named as fronthaul. The fronthaul link connects the remote radio head (RRH) to the baseband (BBU) pool over a common public radio interface (CPRI) separating the base station functions into the processing unit in a centralized location and the radio components located at the cell site 9. Due to the increasing capacity required to support high speed broadband services, the backhaul network is under high pressure.

Figure 3.1: Existing backhaul connectivity in 4G networks 9

To provide bandwidth up to 10 Gbps or beyond with low latency (in terms of microseconds), optical fiber is considered the best solution 10. However, deploying fiber cable is not always possible and may be impossible in some cases and certainly very costly solution. Millimeter wave can play an important role due to fast and low cost deployment compared to optical fiber. So, optical fiber or millimeter wave can fulfill this role, with limitations in terms of deployment cost and limited link path. 11, 12, 13

Figure 3.2: Example of 5G mobile backhaul network consisting of fronthaul and backhaul in hybrid RAN (C-RAN) and (D-RAN )9

Figure 3.3: The purposed 5G cellular mobile backhaul architecture.

3.3. Mobile Backhaul challenges

There are many challenges face mobile backhauling, the most critical one is how to achieve high capacity. For example, if the capacity required for a cell (or sector) in a mobile base station is 1 Gb/s, so the backhaul capacity needed by a 4-sector site should be around 4 Gb/s. In some cases, the mobile traffic from multiple base stations can be aggregated through hub station or aggregated site before reaching the mobile core network. This means much higher data rate required for the mobile backhaul network capacity.
The second important challenge is the link distance of the cellular mobile backhauling. To deliver mobile broadband services to unserved areas, such as rural and remote areas from the main mobile network infrastructure, a long distance backhaul link is required.
The third challenge is how to provide ultra-low latency communications across the mobile backhaul networks to meet the required end-to-end latency. Although low latency is important for the delivery of high quality voice, video and data services, recent applications’ requirements within many industry sectors, especially real time applications critically require ultra-low latency.
The fourth challenge is the cost of the small cell backhaul as a small cell cost should be cheaper compared to the macro base station. For the macro base station backhaul, fiber cable can be used normally for backhauling traffic. This kind of approach solution can’t be used for the small cells backhaul, so new technologies and cost effective solutions are required. 17

3.4. Types of Backhaul/Fronthaul technologies

Existing cellular mobile backhaul networks are usually built with microwave links (operator owned), copper lines, and fiber cables (often leased and sometimes operators owned) with different percentages per operator and per country. The type of backhaul connectivity is a function many parameters such as the location, density of the base stations, line?of?sight (LOS) conditions between the hubs or the aggregation site and base stations, data rate requirements, spectrum license cost and geographical terrain. So, proper backhauling solution can to be chosen according to the network requirements and line?of?sight feasibility. . With the growing of high data rates and network densification demand, the operators are looking for higher bandwidth which can offer much higher data rates. In this section we will summarize the kinds of backhaul/fronthaul technologies which are widely accepted and used by mobile operators and service providers. Backhauling technologies can be divided into two major categories: wired and wireless backhauling. Figure 3.4 shows the classification of cellular mobile backhaul technologies.

Figure 3.4: Different types of backhaul/fronthaul

3.4.1. Wired backhaul networks technologies

Although wired backhaul solutions as copper lines and fiber cable ensure good reliability with large capacity and bandwidth, low latency, but the cost of wired connections is highly dependent on the offered capacity and the link distance.
In addition to that highly reliable wired backhaul connectivity is usually not important for the small cells that are typically serving a relatively reduced traffic load compared to a macro site, even in locations hard to be reached by optical fiber or copper cable.
3.4.1.1. Copper cables

In the past, the leased copper lines dominated the backhaul solutions, as they provided the sufficient capacity per cell site to handle 2G traffic including voice and short message service. Recently, the required backhaul capacity has much increased due to the increasing number of mobile subscribers, the huge increase of mobile high-speed data services and applications, and the significant increase in the number of deployed base station sites. In addition to the limited capacity, the price of copper increases linearly with capacity and distance, thus it is not a cost efficient choice for 5G backhaul network. The alternative to copper cables for mobile backhaul is optical fiber which can give almost unlimited capacity so copper lines are being replaced by optical fibers due to their higher rates/bandwidth and low latency. In this regard, there are two standard Plesiochronous Digital Hierarchy (PDH) hierarchies the T-carriers (T1, T2, T3 and T4) and E-carriers (E1, E2, E3, E4 and E5) 15.

3.4.1.2. Optical Fiber

The optical fiber is the best backhauling medium to offer high capacity meeting all the expectation for traffic increasing thanks to the high bandwidth they offered. But despite its almost unlimited bandwidth, reliability, low latency, low transmission loss, jitter and immunity to electromagnetic interference, in many countries fiber penetration in the backhaul network has been relatively slow compared to microwave solutions, due to the high installation cost and impracticality of laying fiber in difficult terrain (mountains, forests, deserts, jungles…). Deploying a new fiber require a lot of civil works as digging trenches in roads requires permits, traffic management and, once the ducts have been laid, reconstruction of the road by backfilling the hole then reinstating the surface, so owning a fiber is a significantly expensive capital expenditure (CAPEX) option. It is also estimated that leased lines currently account for roughly 15% of the network operating expenditure (OPEX). Optical fiber can travel for long distances, because light propagates through the fiber with much lower attenuation compared to electrical cables. This allows long distances to be divided with few repeaters. 15

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